Distributed optical sensing technology is turning out to be suitable for a number of downhole applications ranging from temperature sensing to passive seismic monitoring. As engineers develop new and improved systems to increase performance and sensitivity, they have encountered certain obstacles. For example, interferometry has proven to be a largely successful approach for enhancing sensitivity, yet it suffers from a phenomenon known as “polarization fading”. When two coherent beams have differing polarizations, they become unable to form the characteristic constructive and destructive interference fringes needed for interferometry. Many fiber installations suffer from unpredictable, dynamic changes in polarization, which are often attributed to small changes in temperature, strain, or just age. In some existing distributed acoustic systems, it is common for around 10% of the sensing channels to be suppressed due to this phenomenon.
Another illustrative obstacle is measurement noise, i.e., noise generated by the sensor and/or receiver electronics. In applications such as passive seismic, the desired signals are extremely weak and may be obscured by this noise.
The conventional solution to these issues is to call for more expensive fiber and receiver electronics. This solution is generally undesirable, particularly for existing installations where it is simply infeasible to replace the existing cable.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.